The present invention relates to a constant current supply circuit for supplying constant current to an electric load via a bipolar transistor.
FIG. 5 shows a constant current supply circuit disclosed in the unexamined Japanese patent publication No. 5-60623. An electric load L has one end connected to a ground terminal and the other end connected to an emitter of a primary transistor Q100. The primary transistor Q100 is an NPN-type transistor which controls the current supplied to the electric load L. A first resistor 101, a second resistor 102, a third resistor 103, and a fourth resistor 104 are serially connected between a high-potential terminal VCC and a ground-potential terminal of a power source. A secondary transistor Q200, being an NPN-type transistor, has a collector connected to a connecting point of the second resistor 102 and the third resistor 103, a base connected to a connecting point of the third resistor 103 and the fourth resistor 104, and an emitter connected to the ground-potential terminal of the power source. A base of the primary transistor Q100 is connected to a connecting point of the first resistor 101 and the second resistor 102. A positive voltage terminal 105, having an electric potential VD higher than the ground potential, is connected to a collector of the primary transistor Q100.
According to this conventional constant current supply circuit, the electric load L receives a constant current (i.e., load current) I from the terminal 105 via the primary transistor Q100. The electric load L has temperature characteristics in its resistance value R. To compensate such temperature characteristics, the relationship among resistance values of the first to fourth resistors 101 to 104 (especially, a resistance ratio of the third resistor 103 to the fourth resistor 104) is determined in such a manner that a voltage E applied between both ends of the electric load L adequately varies in accordance with the temperature. With this setting, the load current I is maintained at a constant value irrespective of temperature change.
However, the above-described conventional constant current supply circuit has the following problems.
It is now assumed that TCRL represents a resistance temperature coefficient of the electric load L, Rtyp represents a typical resistance value of the load resistance R at a reference temperature Ttyp, and .DELTA.T represents a temperature deviation from the reference temperature Ttyp.
Using the above, the load resistance R can be expressed by the formula Rtyp(1+TCRL.times..DELTA.T). The temperature characteristics of the load resistance R is Rtyp.times.TCRL.times..DELTA.T. In other words, the load resistance R causes a variation equivalent to Rtyp.times.TCRL.times..DELTA.T in response to the temperature deviation .DELTA.T from the reference temperature Ttyp. Thus, the temperature characteristics of the electric load L varies in accordance with a change of the typical resistance value Rtyp of the load resistance R.
However, according to the above-described conventional constant current supply circuit, the resistance values of the resistors 101 to 104 are determined in such a manner that the voltage E applied between the both ends of the electric load L varies in accordance with the temperature so as to compensate the temperature characteristics of the load resistance R. Accordingly, the optimum resistance values of the resistors 101 to 104 vary in response to the deviation of the typical value Rtyp of the load resistance R.
Hence, the resistance values of the resistors 101 to 104 need to be adjusted for each electric load L. This forces the workers to perform very complicated adjustment which is not practically feasible.
The electric load L may be a pressure sensing element of a Wheatstone bridge circuit consisting of four strain gauges made of diffused resistors. The resistance value of each diffused resistor has a dispersion range of approximately .+-.10.about.20% due to manufacturing error of the diffusion density of impurities or the width of resistor wire. It is therefore difficult to adopt the above-described conventional constant current supply circuit to this kind of pressure sensor.